Field and Wave Electromagnetics 2nd Edition by David Cheng

Exercise 77

Two very small conducting spheres, each of a mass 1.0 × 10 -4 (kg), are suspended at a common point by very thin nonconducting threads of a length 0.2 (m). A charge Q is placed on each sphere. The electric force of repulsion separates the spheres, and an equilibrium is reached when the suspending threads make an angle of 10°. Assuming a gravitational force of 9.80 (N/kg) and a negligible mass for the threads, find Q.